Telescope 101

There are two basic ways to bring light rays to a focal point.



The earliest method was to bend the rays by passing them through one or more pieces of glass which had curved, polished surfaces, this is called a Refractor.

Advantages of Refractors:

Enclosed tube: dust and moisture doesn’t enter the optical tube.

Fixed optics: don’t require routine collimation  (optics don’t have to be adjusted by the user)

No central obstruction: Less loss of light entering the tube, no alteration of the diffraction pattern

Produce high-contrast, fine-resolution images ideal for planetary viewing

Disadvantages of Refractors:

Many wavelengths of light are passing through glass, uneven bending of the rays causes false color, around bright objects
False color is eliminated with additional lenses and special glass. Requires at least four very accurately shaped, polished and coated lens surfaces
More expensive to produce than other telescope designs

Inexpensive refractors tend to add false color to images



The second method of focusing light is to reflect the rays off of the surface of a curved mirror, producing a type of telescope called a reflector
Most common reflectors in use today are called Newtonians, a design pioneered by Isaac Newton

Rather than lenses, the reflector uses 2 mirrors to bring light to the eyepiece

Advantages of Reflectors:
Dollar for dollar, reflectors offer the most aperture. They produce sharp images that are free of any added color

Disadvantages of Reflectors:

The spider holding the secondary mirror forms a central obstruction that produces a diffraction pattern

Due to the shape of the primary mirror, reflectors suffer from a condition called coma. Objects at the outside edge of the field of view have the appearance of being wedge shaped or look like little comets

Some eyepiece designs work well to counteract this effect

Catadioptric Telescopes:



Catadioptric telescopes are essentially a combination of a refractor and a reflector

There are 2 types of Catadioptrics:
The Schmidt-Cassegrain, and the Maksutov-Cassegrain


Fold the light path 3 times, allowing much shorter tube

No spider and central obstruction, relatively free of optical defects

Focus by moving primary mirror, usually well designed and implemented

Do not require collimation
The real important specifications are as follows:

Stability of the mount … does not vibrate and moves smoothly

Size of aperture … light gathering power

Quality of optics … reflectivity and transmission of light

Ease of use … point and look. The telescope needs a good finder and, in our opinion, especially for the beginner, it MUST have a reflex sighting device of some type

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